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also Figure A8.5). In some cases the fold hinge line, which
generally equates to the fold axis, may be easy to measure, but
if a hinge is not accessible, the following tricks may help.
Where an axial-planar
cleavage intersects a folded
bedding plane, the resulting
intersection lineation (Figure
8.25a) is parallel to the fold
axis. These lineations are
typically easier to fi nd and
measure than the
corresponding fold axis, as
they can occur anywhere on
the fold (Figure 8.25b).
An intersection lineation of cleavage on bedding is typically
visible in the fi eld as thin, dark lines on bedding planes,
whereas bedding on cleavage intersections tends to show up as
stripes of different colour where successive beds intersect the
cleavage plane. Hinges of minor folds formed at the same time
as the major fold may also defi ne a lineation that is parallel to
the major fold axis. This is known as a crenulation lineation
(Figure 8.25c), and is easily measured along the hinges of the
minor folds. An example of how to separate different tectonic
fabrics is provided in Worked Example 8.2.
8
Figure 8.25
Approximations to
fold axes. (a) Thin grooves and
lines (parallel to yellow line) on
dark-grey bedding plane are the
intersection of cleavage with
bedding. Bedding plane dips
steeply to the left. Cleavage traces
(parallel to red line) are visible in
the vertical joint surface on the left
of the photograph, cutting through
steeply-dipping sedimentary layers
(marked with green lines); west
Wales, UK. (b) Diagram showing
relationship of intersection
lineations with a major fold
structure; folded beds are shown in
shades of brown. Intersections of
cleavage on bedding are marked
by dashed yellow lines on the
folded, mid-brown bedding plane;
intersections of bedding planes on
cleavage (marked as dashed red
lines) are shown on the steep
cleavage plane on the left-hand
side. The two lineations are both
parallel to the fold axis. (c)
Crenulation lineation, defi ned by
the hinges of millimetre-scale folds,
on the limb of a larger fold that is
not visible in the fi eld; Strath
Fionan, Scotland. (a and c: Tom
W. Argles, The Open University,
UK.)
Trace of
cleavage
(a)
bedding
plane
fold
axis
fold
axial
plane
exposed
cleavage
plane
cleavage
plane
(
(
